Automatic developability control apparatus

ABSTRACT

A developability control system for a xerographic reproducing machine. The system includes two parallel-spaced NESA glass plates through which two-component developer material, including toner and carrier, flows. The plates are connected in a circuit wherein each is electrically charged alternately for equal periods of time for attracting and repelling toner. A light source is located at one side of the two plates while a photocell is located on the other side to sense the optical density of the sum toner deposit on the two plates at all operating times. This sensing is compared with light received by a second photocell, separated from the same light source by a filter, to cause the dispensing of toner to the developer at appropriate times.

United States Patent Kuhlet a1.

[451 Jan. 18,1972

[54] AUTOMATIC DEVELOPABILITY CONTROL APPARATUS James E. Kuhl; Francis 1). Wltlnski, both of Rochester, N .Y.

Assignee: Xerox Corporation, Rochester, NY.

Filed: Dec. 29, 1969 Appl. No.: 888,727

US. Cl ..222/57, 118/7, 222/75,

Int. Cl ..B67d 5/08 Field ofSearch 222/1, 70, 65,75,57,55, 222/56, DIG. 1; 118/7, 637; 200/6102; 137/93 [56] References Cited UNITED STATES PATENTS Primary Examiner-Robert B. Reeves Assistant Examiner-Larry H. Martin Attorney-James J. Ralabate, Norman E. Schrader and Michael J. Colitz, Jr.

[57] ABSTRACT A developability control system for a xerographic reproducing machine. The system includes two parallel-spaced N ESA glass plates through which two-component developer material, including toner and carrier, flows. The plates are connected in a circuit wherein each is electrically charged alternately for equal periods of time for attracting and repelling toner. A light source is located at one side of the two plates while a photocell is located on the other side to sense the optical density of the sum toner deposit on the two plates at all operating times. This sensing is compared with light received by a second photocell, separated from the same light source by a filter, to cause the dispensing of toner to the developer at appropriate times.

7 Claims, 4 Drawing Figures PATENTED JAN? 8 I972 SHEET 1 [IF 3 w v w m S m m w N s C 3N M MF ATTORNEY PATENIED JAN SHEET 2 III 3 F/GZZ OSCILLATOR HIGH VOLTAGE POWER SUPPLY LAMP 8x PHOTOCELL VOLTAGE SUPPLY LEVEL DETECTOR FIG. 3 I

MOTOR DRIVING STAGE sum 3 [1F 3 PATENI EQ JAN] 81972 AUTOMATIC DEVELOPABILITY CONTROL APPARATUS This invention relates to improvements in sensors and associated controls for toner-dispensing devices. More particularly, this invention relates to improved automatic controls for maintaining the image density constant during the making of electrostatographically produced copy.

Developability, as it pertains to apparatus utilized in graphic reproduction of copying can be defined as the ability of the developer material used in the apparatus to develop to a specified density. A developability control system is one which controls the density of copiesproduced by the apparatus. In the following description of the present invention, the characteristic of developing material which is to be controlled will be its ability to develop images to a predetermined density.

lnthe following description of the present invention, the characteristic of developing material which is to be controlled will be its ability to develop images to a defined density, or in other words the developability of the material. This ability of the material or its developability is often but erroneously considered as related exclusively to the toner concentration of the material, that is, the percent of toner and carrier in the developer mixture. Toner concentration, while the most important aspect in the ability to develop, is only one .aspect.

' Temperature and humidity conditions also affect developability. There are many other factors, such as the state of compaction of the material, the electrical charges on the toner particles and the carrier heads, the state of attraction of toner particles to the carrier bead surface and for that matter, the carrier bead surface wear. For instance, two batches of developer material may have the same toner concentration, however, one batch located in a low-humidity environment will involve developed copy density that is different from the developed copy density of the other batch located in a higher humidity environment. In other words, the developability of the material will be different, even though toner concentration is the same. For the present description then rather than using toner concentration" as the determining factor to be controlled, the all-inclusive term developability will be utilized and since this term is broader in scope, it will be understood that developability" will enhance all that toner concentration suggests.

The present invention avoids the disadvantages of conventional density-sensing devices for controlling the dispensing of toner particles into a developing apparatus used in electrostatic equipment. Generally, the density-sensing method which utilize the periodic collection of toner particles on a NESA glass plate with electrically isolated sections placed in a developing apparatus include the step of sensing light transmission through one. area of the plateas a function of toner concentration. In effect, the plate is developed" with the toner particles, and since a field is placed upon the plate for this purpose, the developing action results in edge development due to the electric field, edge effect. For automatic electrostatic processing wherein large high-speed production runs are frequent, the resultant periodic senses are not efficient as a means of consistent developability indication because of the sudden changes in toner content resulting from high-speed production of randomly presented originals having toner requirements ranging anywhere from heavy, solid area coverage to sparse line coverage.

Various systems have been devised to add toner to a developer mixture of toner and carrier to keep the toner concentration or developability substantially constant within predetermined limits. One such system is described in pending application Ser. No. 867,208 in the name of John Maksymiak. The present invention is an improvement thereafter.

It is therefore an object of this invention to control dispensing in xerographic processing systems such that toner particles will be added to developing material in amounts which are related to the optimum developability for any particular instant of time.

A further object of the invention is to maintain consistency in image quality during xerographic processing.

Another object of the invention is to determine and maintain at all times the proper ratio of toner to carrier in xerographic developing material.

Another object of the invention is to utilize electroded development for collecting toner, and sensing its presence throughout the entire area of the sensing means thereby enhancing the accuracy of each sensing cycle.

Still another object of the invention is dispense toner by a motor and control circuit having two photocells receiving light from a common source, the light for one photocell being a reference signal while the light to the other photocell being a function of optical characteristics of the developer.

These and other objects of the invention are attained by means of a sensor and control means utilized in conjunction with a toner-dispensing device for dispensing toner into a developing apparatus in accordance withthe sensed density of toner alternately deposited on either of two sensor plates. The amount of useful toner particles in developing material can be determined by the amount that will be alternately deposited upon the plates, each being charged with a voltage to set up the proper fields between the plates. This toner determination is utilized to control the amount of active toner within the apparatus. The sensor, having two surfaces, each capable of carrying a charge placed thereon, is positioned within the apparatus to receive some of the developing material falling between the surfaces. Potentials are alternately placed upon the surfaces cyclically thereby reversing the electric field between the surfaces cyclically. This causes toner to be attracted to and cleaned from the surfaces cyclically. While one surface is provided with an attracting field, the other surface serves as a development electrode for the solid area development of the attracting surface since the electric field between the surfaces will be uniform. The amount of toner attracted to each surface when it is charged to attract toner for any particular period of time is related to or a function of the developability in the developer apparatus. The surfaces are coupled in an electrical circuit which produces a first and substantially steady state signal which is compared with a fixed or reference signal so that when the first signal deviates from the reference signal, a control signal is generated to introduce toner particles into the machine toner dispensing system.

Other objects and advantages and further features of the instant invention may be understood when reading the following specification and claims and the accompanying drawings in which:

FIG. 1 is a schematic sectional view of a typical electrostatic reproduction machine embodying the principles of the invention;

FlG. 2 is a sectional view of a toner sensor utilized in the machine shown in FIG. 1;

HQ. 3 is a block diagram of the functional arrangement of a toner-dispensing sensing and control system of the present invention and FIG. 4 is an electrical schematic of the toner concentration sensing elements, toner dispenser motor and control elements therefor.

For a general understanding of a' typical electrostatic processing system in which the invention may be incorporated, reference is had to FIG. 1 in which various components of a typical system are schematically illustrated. As in electrostatic systems such as a xerographic machine of the type illustrated, a light image of a document to be reproduced is projected onto the sensitized member such as a xerographic plate to fonn an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material comprising carrier beads and smaller toner particles triboelectrically adhering thereto to form a xerographic powder image, corresponding to the latent image on the plate surface. The powder is then electrostatically transferred to a support surface to which it may be fixed by a fusing device whereby the power powder image is permanently caused to adhere to the support surface.

The electrostatically attractable developing material commonly used in dry electrostatic printing comprises a pigmented resinous powder referred to here as toner" and a carrier of larger, granular beads formed of glass, sand or steel cores coated with a material removed in the triboelectric series from the toner so that a triboelectric charge is generated between the toner powder and the grandular carrier. The carrier also provides mechanical control so that the toner can be readily handled and brought into contract with the exposed xerographic surface. The toner is then attracted to the electrostatic latent image from the carrier to produce a visible powder image on the xerographic surface while the partially toner depleted carried beads are brought back into the developing system of the machine wherein it is mixed with developing material and, sometimes, a new supply of toner, prior to reuse.

In the illustrated machine, an original to be copied is placed upon a transparent support platen l2 fixedly arranged in an illumination assembly generally indicated by the reference numeral 14. While upon the platen, an illumination system flashes light rays upon the original thereby producing image rays corresponding to the informational areas on the original. The image rays are projected by means of an optical system to an exposure station 16 for exposing the photosensitive layer of a moving xerographic plate in the form of a flexible photoconductive belt 18.

The exposure of the belt to the light image discharges the photoconductive layer in the areas struck by light, whereby there remains on the belt a latent electrostatic image in image configuration corresponding to the light image projected from the original on the supporting platen. As the belt surface continues its movement, the electrostatic image passes through a working zone or developing station 20 in which there is positioned a developer assembly generally indicated by the reference numeral 22 and where the belt is maintained in a flat condition. The developer assembly 22 comprises horizontal and vertical conveying mechanisms which carry developing material to the upper part of the belt assembly whereat the material is dispensed and directed to cascade down over the upwardly moving inclined belt 18 in order to provide development of the electrostatic image.

As the developing material is cascaded over the xerographic plate, toner particles in the development are deposited on the belt surface to form powder images. As the toner powder images are formed additional toner particles are supplied to the developing material in proportion to the amount of toner deposited on the belt during xerographic processing. For this purpose, a toner dispenser generally indicated by reference numeral 24 is used to accurately meter toner to the developer material in the developer assembly 22.

The developed electrostatic image is transported by the belt 18 to a transfer station 26 whereat a sheet of copy paper is moved at a speed in synchronism with the moving belt in order to accomplish transfer of the developed image. There is provided at this station a suitable sheet transport mechanism adapted to transport sheets of paper from a paper-handling mechanism generally indicated by the reference numeral 27 to the developed image on the belt at the station 26.

After the sheet is stripped from the belt 18, it is conveyed into a fuser assembly generally indicated by the reference numeral 28 wherein the developed and transferred xerographic powder image on the sheet material is permanently affixed thereto. After fusing, the finished copy is discharged from the apparatus at a suitable point for collection externally of the apparatus.

It is believed that the foregoing description is sufficient for the purposes of this application to show the general operation of an electrostatic copies for employing a developability control system constructed in accordance with the invention. For further details concerning the specific construction of the electrostatic copier, reference is made to copending application Ser. No. 731,934, filed May 24, 1968 in the name of Hewes et al.

Referring now to FIGS. I and 2, the toner dispenser 24 consists of a hopper or container 30 for the toner particles to be dispensed. Although the hopper or container 30 may be made in any size or shape, the hopper shown is formed as a rectangular open-ended box having tapering side and end walls.

The bottom wall of the hopper 30 may comprise a sliding perforated plate 32 adapted for sliding movement horizontally of the hopper for metering the flow of toner from the hopper. The toner thus dispensed is mixed with the developing material in the developer housing for the apparatus 22 to become almost immediately effective in the developing process. The metering provided by the plate 32 may be controlled by a mechanical device generally indicated by the reference numeral 34, such as a cam plate or linkage system which converts rotary motion of an electrical motor MOTI to reciprocable movement. Preferably, a single revolution of a rotary element in the device 34, say, in the form of a motor shaft, will produce one reciprocable cycle of the plate 32, thereby insuring the dispensing of predetermined quantities of toner. Further details of the plate 32, the linkage system and mechanical device 34 are not necessary to understand the present invention. A preferred form of these devices is illustrated and described in the copending application Ser. No. 731,966 filed on May 24, 1968 in the name ofC. D. Wilson.

In the operation of the toner dispenser, a supply of toner particles is placed within the hopper, the hopper walls and the dispensing plate 32 forming a reservoir for the toner particles. Upon reciprocation of the plate 32, by the device 34, a metered quantity of toner particles will be permitted to enter the apparatus 22. Since the toner dispenser 24 dispenses a uniform quantity of toner for a given stroke length of the metering plate 32, it is apparent that the quantity of toner delivered by the toner dispenser may be varied by varying the number of strokes per actuation of the device 34.

In order to control the dispensing of toner from the toner dispenser 24, there is shown in FIG. 2, an automatic control system which ultimately produces rotation of the rotary element in the device 34 in single-revolution step-by-step operation in accordance with the demands of the control system as it determines the relationship of the developability of the developing material with optimum toner conditions. Basically, the toner-dispensing control system comprises a sensor generally indicated by the reference number 36 mounted within the developer assembly housing 22 by suitable means which electrically insulate the sensor from surrounding structures. Elongated baffle plates 38 are arranged below a horizontal conveyor 40 for the conveyor system for the developer system and are adapted to direct some of the developer material cascading from the conveyor 40 into the developer zone 20. The plates 38 are positioned at angles relative to the vertical and arranged in such a way as to guide developer material falling therebetween into the sensor 36.

The sensor 36 comprises a housing 42 attached to the lower edges of the plates 38 and is formed with a funnel-shaped inlet opening 41 for presenting a circular flow orifice 46 through which entering development material may pass. The diameter of this orifice is such that the rate of flow of developing material through it remains constant during machine operations. Within the housing 42 there is positioned a first sensor plate 48 arranged in a vertical plane and having a generally rectangular configuration. For practical purposes the plate may be of a size having approximately one-half inch for each side. A second sensor plate 50 is also arranged in the housing 42 parallel with the plate 48 and spaced therefrom a short distance. The plates 48, 50 are formed of NESA" glass, a trademark of the Pittsburgh Glass Company, which is generally tin oxide coated glass that is transparent to white light.

The spacing between the plates 48, 50 may be on the order of one-tenth of an inch and is arranged below the flow orifice 46 in the inlet portion 4] of the housing 42. Developing material flows by action of gravity through the flow orifice 46 and between the plates 48 and 50, through the sensor 36 and out of the sensor by way of the outlet portion 52. The material is then conveyed by a duct 54 connected between the outlet 52 and the lower conveyor 55 for the developing apparatus 22 in order to return the material back into the developing system for the machine.

During the operation of the sensor 36, an electric potential of a particular polarity to attract and retain toner particles is applied alternately to the plates 48, 50. As one of the plates is electrically charged to attract toner particles, the other has applied thereto a charge of a polarity which will repel toner particles therefrom during this time. As each of the plates are alternately charged positively and negatively, each plate during a cycle will attract toner for a short periodof time and then immediately repel the same toner. As previously stated, each cycle has an expanse of time preferably on the order of the 1 second whereby, for the first half cycle or for a duration of one-half second, toner particles are attracted and, for the second half of the cycle, toner particles are repelled. During the second half of each cycle, wherein toner particles are repelled, the continuously flowing developing material moving betweenthe plates will clean the particular plate having the repelling charge thereon.

ln the above description, it has been assumed that the toner particles are provided with negative charges so that when either of the plates 48, 50 has applied thereto a proper field, the toner particles will be attracted thereon. .This electrical convention is merely illustrative and has been chosen only for descriptive purposes, The sensor plates function equally well with either positively or negatively charged toner with the same plate potential conditions.

The sensor 36 also includes a first or active photocell P-1 positioned in close proximity to the side of the sensor plate 48 away from the space between the sensor plates. A lamp L-l is also mounted in the sensor 36 and is arranged in close proximity to the side of the sensor plate 50 away from the spacing between sensor plates and in alignment with the plates and photocell P-l. The relative positioning of the photocell and the lamp is such that the photocell will receive the light rays of the lamp through the cascading developer material stream between the sensor plates and the accumulated toner on first one and then the other for each attract and clean cycle. The lamp is connected to a suitable source of electrical power in the control circuit for effecting the energization of the lamp during sensing operation.

Sensing control is accomplished in the present invention by continuously measuring the amount of toner particles that accumulates on both the plates 48, 50 during multiple cycles of attract and clean actions. As previously indicated, a single sensing cycle includes the time when one of the plates 48, 50 attracts toner while the other repels it, and when the other plate attracts while the first attracting plate repels. Therefore, during a sensing cycle, each of the plates 48, 50 attracts toner particles for half the cycle time and each repels toner particles for the other half of the cycle. Cleaning may be accomplished if the plate which is not in the attract mode has a repelling field for the like-charged toner, as determined by the difference in potential between the two plates. For example, cleaning will occur if the clean plate has a potential at ground, or at a negative potential, or at a positive potential, but one which is less than that on the attract plate. if at a negative potential, cleaning will occur if the clean plate is less negative (that is, closer to zero) than the attract plate. As developing material cascades between the sensing plates, the material will clean away the toner particles previously attracted to that plate which is grounded. The effect of this arrangement results in an output of the photocell that is effectively a steady state condition, that is, except for ripples, the output is maintained at a fixed level for the purpose herein. lnsofar as has been described hereinabove, the apparatus and operations are substantially the same as described in the aforementioned application to Maksymiak.

The sensor 36 of the disclosed embodiment of the instant invention is also provided with a second or compensating photocell P-2 located to receive illumination from lamp L-l such that variations in toner concentration in the flow passing through the zone 46 (FIG. 2) will have no effect upon the light received thereat. it is preferably located on the side of the lamp L-l remote from the first or active photocell P-l. The second photocell is of such characteristics and power and located from the lamps such that, when the two photocells are functioning in the yet to be described control circuit, the circuit will function to dispense toner into the developer only when the density of toner flowing through the plates 48 and 50 is outside a desired predetermined limit. This relationship is achieved herein by employing photocells P-l and P-2 of the same characteristics located equidistant from the common source of illumination L-! and by interposing an optical filter 56 between L-l and the compensating photocell P-2. The filter 56 is of such opacity as to correspond to the opacity of the NESA plates during the times when they are electrostatically supporting toner from flowing developer of the desired optimum developability.

The automatic developability control circuit or system functions as an automatic toner dispenser system in that it functions to drive the motor MOT-l to add toner to the developer at appropriate times. The' circuit or system includes the described elements including the, aforementioned NESA plates 48 and 50, the active and compensating photocells P-1 and P-2 the lamp L-l, the toner dispensing motor MOT-l as well as electrical circuitry coupling them to achieve the desired end.

The control circuit, as illustrated in FIGS. 3 and 4 may be considered generally of six major portions or sections, the oscillator section 60, the high-voltage power supply 62, the sensor 64, the level detector 66, the toner dispenser motor driving stage 68 and the lamp and photocell voltage supply 70, hereafter referred to merely as power supply. The sensor section 64 may be considered as being made up of four major parts, the plates 48 and 50, the photocells'Pl and P2, the lamp L-l, and filter 56. The filter is the only-nonelectrical portion of the sensor section 64 and in this sense, the total control circuit may be considered a control system.

The first section to be considered is-the oscillator section 60. The oscillator section consists of a programmable unijunction transistor, Q-14, capacitor C2, resistors R4 and R5, R6 and diode CR2. These elements deliver power to transistor Q6 where the signal is amplified and fed into the base of 04. From 04, the current is picked off in two places to alternately drive each of the high-voltage NESA plate 48 and 50. More specifically, one of the outputs of drives Q7 will clamp the output of 08 when Q4 is on. When O4 is on it also feeds base current to Q9 which will apply low voltage to the plate 48. Q8 and Q9 are portions of the high-voltage power supply and not part of the oscillator as such. The total operation, however, is such that when Q4 goes on, 07 goes on, turning Q8 off. When O4 is on, it also turns on Q9. The resultant action is such that when Q4 goes on, the voltage on plate 50 is applied. When Q4 is off the voltage on Q9 and plate 48 is applied. This is how the switching of the high voltage on a NESA plates 48 and 50 is obtained.

The level detector portion 66 of the circuit consists of elements including the voltage comparator U2. Operation of this device is such that it measures the voltage across the two photocells, the active photocell P1 and the compensating photocell P2. Depending on the relative resistances of these two photocells, the comparator either gives a signal across R33 or it does not. These photocells function as a bridge circuit with resistors R41 and R42. The level of resistance of the photocells, as described above, is an indication of the amount of toner and therefore the developability of the developer. The change in resistance between the photocells indicates whether this developability has changed. The circuit is set up such that when the developability goes down, the signal or voltage on the output of U2 appears at R33. This is the operation of how the level detector takes a level of resistance on a photocell and converts it to a signal which can be used by the motor-driving stage 68. Before describing the motor-driving stage, it is pointed out that the level detector measures only the absolute values of the photocell resistances and not the rates of change. Consequently, variations in the illumination level of the lamp L1 effect both photocells evenly and will not disturb the system.

The motor-driving stage 68 takes the voltage which appears at R33 and uses it to feed the base O10. O10, when it turns on, will pull in a relay K1. K1 then has a contact, Kl-l in .series with the toner motor MOT-l to thereby activate the motor upon closing of Kl-l. When the developability is back to a proper level, the circuit is balanced, U2 will not power R33, Kl ceases to be pulled in, [(1-1 reopens and dispensing of toner ceases through inactivation of motor MOT-l. There is also a developer ready or developer inhibit stage at juncture 23 which prevents the toner motor from driving unless the developer housing is operating. The function of the developer ready, is to prevent the machine from dispensing toner as a result of turning the toner dispenser on except when the developer is running.

The circuit also includes the power supply 70. The power supply 70'provides the current for the lamp sention from transformer T1 and a bridge rectifier B1. The rest of the system is powered by transformer T2 and bridge rectifier B2. At the output of the bridge B2, the power is first sent through the first subportion which is regulated to a predetermined voltage, as 20 volts which will, in turn supply the photocells and other circuit portions. This first subportion consists primarily of Q1, the series pass transistor Q2, the amplifier transistor, and the voltage divider networks R3 and R17. The reference for this particular supply is actually the output of the lamp voltage.

The lamp power supply subportion is powered through T1 and B1 and consists of U1, a voltage regulation module, a voltage divider network consisting of R16, R19 and R29 and also an output stage which is required to increase the capacity of the power supply and consists of Q5 and R14. Filtering for this particular supply is effected by capacitor C6. The high-voltage supply 62 only indicates the electronic switching mechanisms for the NESA Plates 48 and 50 and includes transistors Q8 and Q9. Supply 62 does not include the means of supplying the powering voltage thereto.

The operation of the system is basically as follows. When the machine is turned on, the power supply section 70 is energized through the transformers, the full wave bridge rectifiers and to the two portions of the power supply consisting of the circuit and photocell subportion and also the lamp supply subportion. The lamp is then illuminated and powered by a predetermined voltage, as 3.5 volts. This then drives the photocell P1 to certain levels which indicate the amount of light which is allowed through the NESA plates to the photocell P1 and through the filter to the other photocell P2. The operation of the remaining portion in a standby condition is such that the oscillator section 60 will work although the high-voltage power supply 62 will not switch to voltage on the power supply since the predetermined DC voltage, as 300 volts, supplied externally by source PS-l is not turned on. During this time, the developer ready is at a low potential so that the motor-driving stage could not drive the tonerdispensing motor MOT-1 even if the level detector indicated that there was a lack of toner on the plates. As the machine enters a machine-run condition, the 300 volts is supplied to the high-voltage power supply and the oscillator, turning on alternately Q8 and Q9 will switch the voltages from NESA plates 48 and 50. This allows the toner to be built up and be cleaned off during prescribed periods which is determined by the frequency of oscillation of the oscillator. The photocell Pl will now sense the amount of light coming from the plates and determine whether or not there is sufficient to cause the level detector U2 to have an output signal. When the machine is running and the developer housing is running, the developer ready condition is high such that a signal coming from the level detector can be interpreted and acted upon by transistor 0-10 and the motor-driving stage will in turn close relay Kl which is in series which the toner dispensing motor MOT-1 to .thereby dispense toner to the developer as required.

From the foregoing it will be apparent that the sensor 36 is, in effect, a miniature cascade-developing device wherein the development of either of the plates 48 and 50 is enhanced by the other plate serving as a development electrode. Development is effected by means of an electrode field and not by fringe fields which occur when a single surface is employed without the influence of a parallel-spaced surface having a charge for producing an electric field therebetween. The development then on each of the plates 48 and 50 is complete; that is, the distribution of toner throughout the plate is uniform. in other words, the entire sensing surface is essentially utilized for development thus providing a larger relative area from which density may be measured which in turn allows better density averaging. The photocell Pl may then be applied to the entire sensing surface area instead of merely the conventional small area utilized for scanning or sensing. This sensed area will then be electrically utilized in conjunction with the area sensed by photocell P2 to define developability of the sensed developer and for adding toner thereto at appropriate times.

While the invention has been described with reference to the structure disclosed, it is not confined to the details set forth; but is intended to cover such modifications, or changes as may come within the scope of the following claims.

What is claimed is:

l. A toner-dispensing control device for use in an electrostatic reproducing apparatus having a photosensitive plate and a developing system adapted to apply electrically charged toner from a developing material to an electrostatic latent image recorded on the plate thereby producing powdered toner images thereon including:

plate means arranged to contact developing material to be controlled for developability, said plate means including a pair of spaced-apart, transparent, conductive electrodes,

means for cyclically producing an electrical charge on first one and then the other of the electrodes capable of attracting toner thereon whereby the toner is attracted from the developer material to one of said electrodes and, simultaneously therewith, released from the other of said electrodes,

a light source,

means for directing light rays from the light source through both of said electrodes and the toner attracted thereto,

a first light-sensitive device arranged to receive light rays projected through both said electrodes and to produce a signal in accordance with the amount of toner present thereon during repeated sensing cycles,

a second light-sensitive device positioned remote from said first light-sensitive device to receive light rays from said light source that are not projected through said plate means to produce a signal in accordance therewith,

circuit means for coupling said first and second light-sensitive devices for comparing the output signals thereof, and

means responsive to said circuit means to dispense toner into the developer system at the occurrence of a predetermined condition determined by said circuit means.

2. The apparatus as set forth in claim 1 and further including a light filter having an optical density indicative of the optimum developability of the developing system positioned between said light source and said second light-sensitive device.

3. The apparatus as set forth in claim 1 wherein said first and second light-sensitive devices are positioned equally distant from the light source.

4. Apparatus for controlling the developability of a developing system having a toner-dispensing means associated therewith for adding toner to a developer material made up of carrier granules and toner particles including a pair of parallel-aligned and spaced-apart transparent electrodes, the electrodes being positioned between a source of light and a first light-sensing means, said electrodes being arranged to support a flow of developer material therebetween,

means for cyclically producing a toner-attracting charge on first one of said electrodes and then on the other of said electrodes whereby toner is attracted to one of said electrodes to develop said electrode and, simultaneously therewith, released by the other of said electrodes,

said first light-sensing means being capable of generating a continuous electrical output signal indicative of the amount of light passing through the two electrodes as the electrodes are cyclically developed with toner,

a second light-sensing device positioned in light-receiving relation with said light source and producing an electrical output signal 'in accordance with the amount of light received,

a filter interposed between said second light-sensing device and said source having an optical density whereby said second light-sensing source generates an output signal indicative of the optimum developability of the developing system, and

circuit means to compare the outputs of said first and second light-sensing means and actuating the toner dispenser in response thereto when the level of developability has deviated from a predetermined level.

5. The apparatus as set forth in claim 4 wherein said first and second light-sensing means are photocells.

6. The apparatus of claim 4 wherein said first and second sensing means are positioned an equal distance from the light source.

' 7. The apparatus of claim 4 wherein a toner-repelling charge is cyclically produced on said other electrode whereby said other electrode is cleaned of toner at about the same rate as said one electrode attracts toner.

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1. A toner-dispensing control device for use in an electrostatic reproducing apparatus having a photosensitive plate and a developing system adapted to apply electrically charged toner from a developing material to an electrostatic latent image recorded on the plate thereby producing powdered toner images thereon including: plate means arranged to contact developing material to be controlled for developability, said plate means including a pair of spaced-apart, transparent, conductive electrodes, means for cyclically producing an electrical charge on first one and then the other of the electrodes capable of attracting toner thereon whereby the toner is attracted from the developer material to one of said electrodes and, simultaneously therewith, released from the other of said electrodes, a light source, means for directing light rays from the light source through both of said electrodes and the toner attracted thereto, a first light-sensitive device arranged to receive light rays projected through both said electrodes and to produce a signal in accordance with the amount of toner present thereon during repeated sensing cycles, a second light-sensitive device positioned remote from said first light-sensitive device to receive light rays from said light source that are not projected through said plate means to produce a signal in accordance therewith, circuit means for coupling said first and second light-sensitive devices for comparing the output signals thereof, and means responsive to said circuit means to dispense toner into the developer system at the occurrence of a predetermined condition determined by said circuit means.
 2. The apparatus as set forth in claim 1 and further including a light filter having an optical density indicative of the optimum developability of the developing system positioned between said light source and said second light-sensitive device.
 3. The apparatus as set forth in claim 1 wherein said first and second light-sensitive devices are positioned equally distant from the light source.
 4. Apparatus for controlling the developability of a developing system having a toner-dispensing means associated therewith for adding toner to a developer material made up of carrier granules and toner particles including a pair of parallel-aligned and spaced-apart transparent electrodes, the electrodes being positioned between a source of light and a first light-sensing means, said electrodes being arranged to support a flow of developer material therebetween, means for cyclically producing a toner-attracting charge on first one of said electrodes and then on the other of said electrodes whereby toner is attracted to one of said electrodes to develop said electrode and, simultaneously therewith, released by the other of saiD electrodes, said first light-sensing means being capable of generating a continuous electrical output signal indicative of the amount of light passing through the two electrodes as the electrodes are cyclically developed with toner, a second light-sensing device positioned in light-receiving relation with said light source and producing an electrical output signal in accordance with the amount of light received, a filter interposed between said second light-sensing device and said source having an optical density whereby said second light-sensing source generates an output signal indicative of the optimum developability of the developing system, and circuit means to compare the outputs of said first and second light-sensing means and actuating the toner dispenser in response thereto when the level of developability has deviated from a predetermined level.
 5. The apparatus as set forth in claim 4 wherein said first and second light-sensing means are photocells.
 6. The apparatus of claim 4 wherein said first and second sensing means are positioned an equal distance from the light source.
 7. The apparatus of claim 4 wherein a toner-repelling charge is cyclically produced on said other electrode whereby said other electrode is cleaned of toner at about the same rate as said one electrode attracts toner. 